Processes for making 2,2'-oxodisuccinate in metal salt form are known and disclosed in the art. Berg, U.S. Pat. No. 3,128,287, issued Apr. 7, 1964, states that the process of his invention comprises an efficient method of producing 2,2'-oxodisuccinic acid and malic acid and provides the following disclosure:
(Col. 1, lines 32-39)
The process of this invention involves the reaction of maleic acid with a hydroxide of calcium, barium, magnesium or strontium. In general this reaction is conducted by admixing maleic acid with an excess of the hydroxide in the presence of water. The reaction mixture is then heated for from about one day to about one month at temperatures ranging from about 50.degree. C. to reflux temperatures.
See also Col. 1, lines 65-71, which state:
As indicated above, the process of this invention produces both malic and 2,2'-oxodisuccinic acid, the ratio of these products varies with the metal hydroxide employed in the process. When strontium and barium hydroxides are employed, an almost quantitive [sic] conversion of maleic to malic acid can be effected. However, the use of calcium and magnesium hydroxide in this process produces almost an equal mixture of malic and 2,2'-oxodisuccinic acids.
See also the examples, especially Example 1.
Regardless of the efficiency of the Berg process for producing malic acid or "almost equal" mixtures thereof with 2,2'-oxodisuccinate, a synthesis directly leading to high yields of 2,2'-oxodisuccinate, with only low levels of malate or organic byproducts such as fumarate, is not disclosed. Thus, if it is desired to isolate the 2,2'-oxodisuccinate salts or formulate them into a detergent composition without automatically co-introducing substantial amounts of malate into the formulation, organic purification of the product is required. Low yield and low organic purity of the 2,2'-oxodisuccinate crude product of the Berg invention render its large-scale production commercially unattractive.
A 2,2'-oxodisuccinate synthesis process based upon Berg is also disclosed by Lamberti et al., U.S. Pat. No. 3,635,830 issued Jan. 18, 1972. The Lamberti et al. process shares the disadvantages of the Berg process.
Lamberti et al. further discloses that 2,2'-oxodisuccinate salts are useful laundry detergent builders.
Matzner et al., Tenside Detergents, 1973, Vol. 10, 239-245, writing in the context of synthesis of 2,2'-oxodisuccinates as detergent builders, state, "A more economical process would have to be developed to make this a practical (builder) candidate."
Notwithstanding the more recent disclosure of Nieuwenhuizen et al, J. Amer. Oil Chemists' Soc., Vol. 60, 1983, pages 44-48, that it is possible to conduct a laboratory synthesis of 2,2'-oxodisuccinates by addition of (preformed) malic acid to maleic acid in aqueous alkaline medium in the presence of a divalent cation, preferably Ca.sup.2+, no commercially viable industrial process for making 2,2'-oxodisuccinates appears to have been disclosed in the art.
The Berg and Lamberti et al. patents cited above are incorporated herein by reference, especially insofar as they also include disclosure of conversion of crude calcium or magnesium-containing product of the above-cited processes to 2,2'-oxodisuccinate monovalent cation salts (e.g., sodium salts). As noted, separation of 2,2'-oxodisuccinate from other organic species such as malate is also disclosed.
In light of the foregoing, there is a clear need for an improved 2,2'-oxodisuccinate-forming process. A high-yield process, which is capable of improving the economics and/or practicality of 2,2'-oxodisuccinate production, would be especially useful to the user and manufacturer of detergent compositions. A cost-effective, large-scale process should very preferably minimize the need for organic purification of the crude 2,2'-oxodisuccinate product.
It is an object of the present invention to provide such an improved 2,2'-oxodisuccinate synthesis process, wherein the ether-bond forming reaction: EQU maleate+malate.fwdarw.2,2'-oxodisuccinate
is carried out efficiently and in high yield.
It is a further object of the invention to provide a process which is usable in an overall conversion of maleic anhydride to 2,2'-oxodisuccinic acid or sodium salts thereof. Yet another object of the invention is the provision of a process for cost-effectively preparing laundry-detergent grade 2,2'-oxodisuccinate salts on a large scale, wherein organic purification is optional rather than essential.